The present invention relates to improvements afforded to gas appliances, and especially to gas water heaters, comprising a gas burner placed in the lower part of the said appliance and in the vicinity of a surface underlying this appliance, which further comprises an atmosphere control pilot light connected to the gas supply to the burner and means for detecting the temperature of the flame of the said pilot light which are operationally coupled to means for cutting off the gas supply to the main burner.
In some countries (especially North America: USA and Canada), gas water heaters of this type equip not only individual dwellings, but also industrial premises such as garages or workshops for repairing motor vehicles. In the latter type of installation, very many accidents due to ignition of petrol vapours unintentionally sprayed in the immediate vicinity of the base of the appliances, where the gas burner is found, have been noted.
To prevent such accidents, the appliances have been modified and fitted with fireproof netting in front of the primary air intakes needed for the operation of the burner. However, this solution is not satisfactory: given the dirty and dusty environment in the locations where the appliances are sited, the netting, which must have fairly fine mesh, clogs up relatively quickly and obstructs the primary air intake: this then results in a high increase in the CO content and a serious risk of asphyxia.
In other cases, the appliances have been made with sealed combustion chambers supplied with primary air by vertical pipes, the inlet orifices of which are raised with respect to the surface (in general the ground) on which the appliance rests and on which there is a risk of the liquid petrol being spilt with the formation, above, of stagnant petrol vapours. Here again this solution is not satisfactory because it involves modifying the basic structure of the appliances.
The aim of the invention is to provide an improved solution which is particularly efficient from the point of view of safety, which is suitable for a dirty environment, which requires no modification to the basic structure of the appliances and which, as a result, can be applied not only to new appliances, but also to already existing appliances by means of a simple modification, and, finally, which does not involve too high an additional cost.
To these ends, a gas appliance as mentioned in the preamble is characterized, being arranged according to the invention, in that the atmosphere control pilot light comprises a primary air inlet orifice connected to which is a tube, the free orifice of which is located below the gas burner.
By virtue of such an arrangement, the gas supply to the main burner is interrupted when the temperature detection means detect cooling of the flame of the pilot light which is caused by gas enrichment of the mixture (presence of vapours in the primary air or obstruction of the air intake by a liquid phase, especially liquid petrol).
In a preferred manner, the free orifice of the said tube is located in the immediate proximity of the surface supporting the appliance, that is to say that it is located as close as possible to this surface in order to ensure that the safety device will act as soon as the vapours and/or a liquid (petrol) appear on the underlying surface, while however leaving a passage which is large enough so that sufficient primary air can be sucked up and that the pilot light can operate normally in the absence of petrol. In particular, in a normal installation mode, the appliance rests on the ground and the free end of the tube is then located in the immediate proximity of the ground.
In a simple exemplary embodiment, the means for detecting the temperature of the flame comprise at least one thermocouple which is placed in the axis and downstream of the pilot light and the output voltage of which controls the means for cutting off the gas supply to the main burner.
In an efficient embodiment making it possible to overcome the influence of the ambient temperature, the means for detecting the temperature of the flame comprise two thermocouples, it being possible for the second of the thermocouples to be offset laterally with respect to the axis of the pilot light, which are suitable for generating a non-zero output voltage in the presence of a flame resulting from correct operation of the pilot light and an output voltage which is substantially zero in the absence of a flame or in the presence of an abnormal flame due to gas enrichment of the mixture, and in that the means for cutting off the gas supply are of the type which open in the presence of a non-zero control voltage and which close in the presence of a zero control voltage.
In another embodiment, which makes it possible to obtain enough voltage to excite the means for controlling the gas supply to the main burner, provision is made for the means of detecting the temperature of the flame to further comprise a thermal microswitch, which may be placed laterally offset with respect to the axis of the pilot light, which has an input terminal connected to an electrical supply and an output terminal connected to the means for controlling the gas supply to the main burner, and which has an excitation input terminal connected to the abovementioned thermocouple (12), for the said microswitch to be suitable for generating an output voltage equal to the supply voltage when it is excited by the thermocouple put in the presence of a flame resulting from correct operation of the pilot light and a zero output voltage in the absence of a flame or in the presence of an abnormal flame due to gas enrichment of the mixture, and for the means for controlling the gas supply to be of the type which open in the presence of a non-zero control voltage and which close in the presence of a zero control voltage.
In a variant embodiment suitable for preventing inadvertent shutdown of the main burner, provision is made for a moveable screen sensitive to a temperature increase to be inserted between the second thermocouple or the microswitch and the pilot light, such that the said screen remains in position in the presence of a stable or briefly elongated flame so as to prevent inadvertent triggering of the said second thermocouple or microswitch and to prevent the gas supply being inadvertently cut off, but that the said screen is moved under the action of heating due to a flame which is elongated in a lasting manner and then sets the said second thermocouple or microswitch in the presence of the said elongated flame so that it cuts off the gas supply. The screen may then advantageously consist of a bimetallic strip.
As emerges clearly from the above explanations, the provisions of the invention have a particularly beneficial, although not exclusive, application in gas water heaters.